1. Field of the Invention
The present invention relates to light intensity control apparatus and method, in which light emitted from a light source is split into monitor light and principal light so that the quantity of light to be emitted from the light source can be controlled in accordance with the intensity of the monitoring light.
The present disclosure relates to subject matter contained in U.S. patent application Ser. No. 08/654,824 filed on May 29, 1996 entitled "Light Intensity Controlling Device", now U.S. Pat. No. 5,892,219, the disclosure of which is expressly incorporated by reference herein.
2. Description of the Related Art
In an optical scanner or the like, the light emitted from the light source is split into monitor light and principal light and the intensity of the monitor light is detected, so that the quantity of the light to be emitted from the light source can be controlled in accordance with the intensity of the monitor light to thereby control the intensity of the principal light. For example, in an optical disc apparatus, laser beams emitted from a light source such as a semiconductor laser is split by a beam splitter such as a half mirror into monitor light which is used to detect the light intensity and principal light which is made incident upon an optical disc located on an image forming surface to read data therefrom or write data thereon.
In the intensity control apparatus, a so-called APC (Automatic Power Control) operation is carried out in which the light intensity of the light source is controlled in accordance with the light intensity of the monitor light to obtain a desired light intensity on the optical disc. To this end, the intensity of the output signal of the optical detector which detects the monitor light must have a predetermined correlation with the light intensity of the principal light. However, since the reflection-transmission characteristics of the beam splitter or other optical elements located behind the beam splitter vary depending on the polarization (polarization dependency), if the polarization state of the light incident upon the beam splitter changes, the balance between the output signal intensity of the monitor light and the light intensity of the principal light is broken, thus resulting in the APC operation not being carried out effectively.
For instance, in an optical apparatus in which a plurality of light sources are employed, the polarization state of the plural light bundles tends to be irregular. Moreover, in case of the light emitted from a single light source, the polarization state of the light varies due to environmental conditions or the arrangement of the optical elements before the light reaches the beam splitter. If a change in the polarization state of the incident light occurs, it is impossible to correct the output signal since the optical elements including the beam splitter have dependency on the polarization. Consequently, even if the APC operation is carried out to control the quantity of light to be emitted from the light source, it is impossible to keep the light intensity of the principal light on an object surface at a predetermined level. In the worst case, an intensity error of the light on the object surface could be enhanced by the APC operation.
In addition, the polarization dependency characteristics (polarized light component ratio of the reflection light or transmission light) of the beam splitter vary depending upon the humidity. For instance, since the surface of the half mirror coated with a dielectric layer is exposed to the outside, the polarized light component ratio of the monitor light transmitted through the half mirror tends to change due to a change in the humidity of the ambient air. Therefore, it is desirable that the light intensity control be carried out, taking into account influence of a change in humidity.